Fuel control apparatus for gas turbine engine

ABSTRACT

A fuel control apparatus for a gas turbine engine reheat system which includes a plurality of burners has a plurality of variable metering orifices associated with the respective burners and a plurality of servo-responsive throttle valves downstream of the respective orifices. The apparatus includes an arrangement for controlling the servo pressures applied to the throttle valves, this arrangement including a governer driven by the engine and mounted on an internal ring gear. A first valve spool is coupled to the ring gear and is axially movable by the governer. The ring gear includes a coaxial spur gear which meshes with further spur gears forming parts of respective valve spools which control the servo pressures for associated throttle valves.

Skinner 5] July 23, 1974 I 1 FUEL CONTROL APPARATUS FOR GAS TURBINEENGINE [75] Inventor: Robert Thomas John Skinner, High Wycombe, England[73] Assignee: Lucas Aerospace Limited,

Birmingham, England 22 Filed: Sept. 25, 1972 21 Appl.N0.:291,604

Primary Examiner-Clarence R. Gordon Attorney, Agent, or Firm-Holman &Stern [57] ABSTRACT A fuel control apparatus for a gas turbine enginereheat system which includes a plurality of burners has a plurality ofvariable metering orifices associated with the respective burners and aplurality of servoresponsive throttle valves downstream of therespective orifices. The apparatus includes an arrangement forcontrolling the servo pressures applied to the throttle valves, thisarrangement including a governer driven by the engine and mounted on aninternal ring gear. A first valve spool is coupled to the ring gear andis axially movable by the governer. The ring gear includes a coaxialspur gear which meshes with further spur gears forming parts ofrespective valve spools which control the servo pressures for associatedthrottle valves.

31 Claims, 3 Drawing Figures FUEL CONTROL APPARATUS FOR GAS TURBINEENGINE This invention relates to an improvement in a fuel controlapparatus for a gas turbine engine reheat sys-, tem of the kinddescribed in our co-pending US. Pat. application No. 239,584 now US.Pat. No. 3,777,483

granted Dec. 11, 1973.

According to the invention a fuel control apparatus for a reheat systemof the foregoing kind comprises a first variable metering orificeincluding a control member movable to control fuel flow to a firstreheat burner in accordance with a first engine operating parameter,second and third variable metering orifices to control fuel flow tosecond and third reheat burners respectively, said second and thirdorifices each including a control member movable in response to theposition of the control member of the first metering or orifice and alsoin response to second and third engine operating parameters, throttlesbetween the metering orifices and their associated burners, the throttleassociated with the first metering orifice being responsive to a firstservo pressure dependent on the pressure downstream of both the secondand third metering orifices, and the throttles respectively associatedwith the second and third metering orifices being response to second andthird servo pressures respectively dependent on the pressure downstreamof the associated metering orifices, and an arrangement for controllingthe said servo pressures in accordance with the said downstreampressures and with the speed of the engine, the said arrangement havingan internal ring gear, a first spur gear driven, in use, by the engineand meshing with the ring gear, a speed responsive governor driven bythe ring gear, a valve spool rotatable by the ring gear and axiallymovable by the governor, a second spur gear secured coaxially to thering gear, a plurality of further valve spools respectively axiallymovable in response to said downstream pressures and a plurality offurther spur gears respectively secured to said further spools andmeshing with said second spur gear.

A fuel control apparatus according to the invention will now bedescribed by way of example and with reference to the accompanyingdrawings in which:

FIGS. 1 and 2 show the apparatus diagrammatically, and

FIG. 3 shows, somewhat diagrammatically, the gear arrangement, viewed online 3-3 in FIG. 2.

The apparatus has a centrifugal vapour core fuel pump driven by theengine. Pump 10 is of the kind in which fuel delivery is controlled by athrottle responsive to pump delivery pressure and to a servo pressuresignal applied via a passage 12 to oppose the pump delivery pressure.This servo pressure signal is derived in a manner later to be described.

A metering assembly, shown generally at 13, has an inlet 14 connected tothe outlet of pump 10 and includes three variable metering devices 15,l6, l7 arranged in parallel to control fuel flow to a primary burner, agutter and a collander respectively of the engine reheat system.

Device 15 comprises a sleeve 18 within which a valve spool 19 isslidable and rotatable. Sleeve 18 has ports 20 which communicate withinlet 14 and also has a further port 21 which communicate with anannular passage 22. Valve spool 19 includes a recess 19a by means ofwhich fuel can flow between ports 20, 21. Valve spool 19 is rotatable byan electric actuator 23 responsive to a temperature T, at the engine airintake. Valve spool 19 includes a piston portion 24 slidable in acylinder 24a and subjected on opposite sides to servo pressures derivedfrom the pressure at the inlet 14. One side of piston 24 communicatesvia a valve 25 with a chamber 36 and also via a restrictor 26 with inlet14. The other side of piston 24 communicates via a valve 27 with chamber36 and also via restrictor 28 with inlet 14. Valves 25, 27 haveassociated half ball closure members 29, 30 mounted on a single controllever 31 by a bellows 32 in response to a pressure P 1 derived from thedelivery pressure P of the engine compressor, pressure P P beingproportional to pressure P; when downstream orifice 120 is choked.

A spring 33 is engaged between lever 31 and the adjacent end of spool19, and a spring 34 is engaged between lever 31 and an adjustable stop35 on the assembly 15. Chamber 36 communicates with a low pressuredrain.

The pressure DP, in the annular passage 22 surrounding ports 21communicates via a restrictor 37 and a passage 38 with a chamber 39 in aservo pressure control arrangement, shown generally at 40. Annularpassage 22 also communicates via a throttle valve 41 and an annularpassage 42 with the primary burners of the reheat system. Throttle valve41 includes a metering orifice 43 and a piston control element 44 biasedagainst the pressure in passage 22 by a spring 45 and a servo pressuresignal S obtained, in a manner later to be described, from the servocontrol arrangement 40 and supplied via a passage 46.

Device 16 includes a sleeve 47 formed with a port 48 which communicateswith inlet 14. A piston control element 49 (shown in greater detail inFIGS. 2 and 3) is slidable and rotatable in sleeve 47 and includes athrough port 50 which combines with port 48 to provide a variablemetering orifice. Element 49 defines within sleeve 47 a chamber 51 whichcommunicates with inlet 14 via a restrictor 52. Element 49 includes astem 53 having an axial bore 54 which communicates with chamber 51. Aball closure member 56 is captive on stem 53 and is operable to shut offthe end of bore 54.

Valve spool 19 of device 15 has secured thereto a three-dimensional cam57. A cam follower 58 is biased into engagement with cam 57 by a spring59 and is also engageable with ball closure member 56. An edge cam 60 isrotatable by a linkage 61 in accordance with the position of a powerdemand control for the engine. A cam follower 62 engages cam 60 and issecured to stem 53 to rotate control element 49 in accordance with theposition of the said power demand control.

Downstream of a device 16 is a throttle valve 63 including a meteringorifice 64 and a piston control element 65 biased against the pressuredownstream of device 16 by a spring 66 and by a servo pressure signal 52obtained from the servo control arrangement 40 and supplied via apassage 67. Downstream of metering orifice 64 is a passage 68 throughwhich fuel can reach the gutters of the reheat system. The pressure DPimmediately downstream of device 16 is supplied via a restrictor 69 anda passage 70 to the arrangement 40 to control the servo pressure signal52 in a manner later to be described.

Device 17 is identical with device 16, having a piston control element71 axially and rotatably positioned by cams 57 60 respectively. Anassociated throttle valve 72 responsive to a servo pressure signal 53supplied via a passage 73 from control arrangement 40 and dependent onthe pressure DP immediately downstream of device 17. Pressure DP; issupplied via a restrictor 74 and passage 75 to the arrangement 40. Thedownstream side of throttle valve 72 communicates via a passage 76 witha collander of the engine reheat systern.

Cam follower 62 and an identical cam follower 77 associated with device17 are biased into engagement with cam 60, by means of a rod 78interconnecting followers 58, 77 and a plurality of spring washers 79mounted in the rod.

Arrangement 40 has a first valve spool 80 urged in one direction by thedelivery pressure of pump and in the opposite direction by the pressureDP, in chamber 39 and by a governor mechanism 81 responsive to enginespeed. Valve spool 80 is slidable within a sleeve 82 having a port 83 towhich is applied a fluid from a high pressure source. This fluid isconveniently the fuel supplied by a high pressure pump for the engine. Afurther port 84in sleeve 82 communicates with a low pressure drain, anda port 85, intermediate ports 83, 84 communicates with passage 12. Anincrease in the delivery pressure P of pump 10 causes valve spool 80 tobe urged to the right, as seen in FIG. 1, shutting off port 83 andopening port 84, The pressure in passage 12 thus falls and throttle 11moves to reduce fuel flow and hence delivery pressure of pump 10. Anincrease in engine speed or a rise in pressure DP, downstream ofvariable metering device 15, similarly causes the delivery pressure ofpump 10 to be increased. Pump delivery pressure P, at inlet 14 is thusvaried to maintain the difference between pressure DP, and Psubstantially constant for a given engine speed, this pressuredifference being variable with engine speed. The weights 81a of governormechanism 81 have a relatively low specific gravity, whereby, when adenser fuel is used, the effect of engine speed on valve spool 80 isreduced.

Servo control arrangement 40 also includes a valve 86 having a spool 86aresponsive to pressure DP and to the opposing pressure DP obtained asabove described from metering assembly 13. Pump delivery pressure P, isapplied to an inlet 87 of valve 86. Outlets 88, 89 respectivelycommunicate with a low pressure drain and with passage 46 in meteringassembly 13. Sppol 86a includes a portion movable with respect to afurther outlet 90 to provide the servo pressure 52, outlet 90communicating with passage 67 in metering assembly 13. A fall inpressure DP causes spool 86a to move so as to increase the value ofpressure 52 and thereby to urge control element 65 to reduce meteringorifice 64, restoring pressure DP A rise in pressure DP, movesl78 spool86a in the same direction, causing pressure DP to rise to the same valueas pressure DP,. Outlet 89 is noramlly shut and communicates withpassage 46 and also via a restrictor 91 with a low pressure drain. If arise in pressure DP or a fall in pressure DP, causes a change inpressure S which is insufficient to bring DP, equal to DP, then outlet89 opens. The resultant increase in servo pressure 5, causes throttle 41to close so as to tend to restore pressure DP, to the level of DP In asimilar manner an increase and decrease in pressures DP, and DPrespectively moves spool 86a in a direction to restore the originalpressures. The arrangement is such that pressures DP, and DP aremaintained substantially equal to each other.

Servo control arrangement 40 also includes a further spool valve 92,substantially identical to valve 86, but responsive to pressures DP andDP;. Spool valve 92 operates in the same way as valve 86 to maintainpressure DP equal to pressures DP, and DP Spool and the spools of valves86, 92 are driven by a gear train powered by the engine. The gear traincomprises an internal ring gear 93 with which a spur gear 94, driven bythe shaft of pump 10, is in mesh. Ring gear 93 has an axial boss formedexternally as a spur gear 95 and internally with splines 96 which engagecomplementary splines on spool 80. The weights 81a of governor 81 arecarried on a cage 97 integral with ring gear 93. The spools of valves86, 92 are provided with respective spur gears 98, 99, both of whichmesh with spur gear 95, as shown in FIG. 2, the arrangement shown inFIG. 1 being for the purpose of clarity only.

In use, fuel is supplied from pump 10 to the metering assembly 13, andpasses via device 15 and throttle valve 41 to the passage 42. A separateshut-off cock and ignition arrangement (not shown) prevent fuel fromreaching the primary burners unless reheat is selected. Fuel flow to theprimary burners is controlled by axial movement of valve spool 19 inresponse to pressures P and P P. Spool 19 is rotated by actuator 23 inaccordance with temperature at the engine compressor intake. Rotation ofspool 19 has no effect on primary flow but positions thethree-dimensional cam 57. Cam 57 is formed with a series of profileswhich correspond to functions of T for a number of values of P Controlelement 49 is urged upwards, as seen in FIG. 1, in a direction to reducefuel flow, by the pressure downstream of the metering orifice formed byports 48, 50. This upward movement is resisted by the pressure inchamber 51. Pressure in chamber 51 is controlled by ball closure member56. If ball 56 is permitted to open by cam follower 58, pressure inchamber 51 falls and element 49 moves to reduce fuel flow. With ball 56shut, pressure in chamber 51 is greater than that downstream of ports48, 50, and element 49 moves to increase fuel flow. Element 49 thus actsas a follow up servo piston responsive to the position of cam follower58, which is in turn dependent on the value of P and the function of Tderived from the cam 57. Element 49 is also rotated in response to theposition of the power demand control for the engine, port 50 being movedtowards or away from, register with port 48 as element 49 is rotated.Element 71 of device 17 operated, in the same way as element 16, tocontrol fuel flow to the reheat collander.

The output pressure of the pump 10 is controlled as previously describedto maintain pressure DP, substantially constant for a given engine speedand fuel density, and pressures DP and DP;; are maintained substantiallyequal to DP,. As a result, fuel flows to the reheat burners are alsoresponsive to engine speed, N, flow to the primary burners beingdependent therefore on the values N and P Flows to the gutter andcollander are dependent on the values N and P obtained as a result ofaxial position of spool 19, on the value of functions of T obtained fromthe cam 57, and on the value of obtained via cam 60.

Rotation of the valve spools of control arrangement 40 prevents theoperation of this arrangement being adversely affected by staticfriction. The use of ring gear 93 both as an element of the train and asa carrier for the governor mechanism 81 permits a substantial saving inweight and space.

I claim:

1. A fuel control apparatus for a gas turbine engine reheat system whichincludes a plurality of burners, comprising a first variable meteringorifice including a control member movable to control fuel flow to afirst reheat burner in accordance with a first engine operatingparameter, second and third variable metering orifices to control fuelflow to second and third reheat burners respectively, said second andthird orifices each including a control member movable in response tothe position of the control member of the first metering orifice andalso in response to second and third engine operating parameters,throttles between the metering orifices and their associated burners,the throttle associated with the first metering orifice being responsiveto a first servo pressure signal dependent on the pressures downstreamof both the second and third metering orifices, and the throttlesrespectively associated with the second and third metering orificesbeing responsive to second and third servo pressure signals respectivelydependent on the pressures downstream of the associated meteringorifices, and an arrangement for controlling said servo pressure signalsin accordance with said downstream pressures and with the speed of theengine, said arrangement having an internal ring gear, a first spur geardriven, in use, by the engine and meshing with the ring gear, a speedresponsive governor driven by the ring gear, a valve spool rotatable bythe ring gear and axially movable by the governor, a second spur gearsecured coaxially to the ring gear, a plurality of further valve spoolsrespectively axially movable in response to said downstream pressuresand a plurality of further spur gears respectively secured to saidfurther spools and meshing with said second spur gear.

2. An apparatus as claimed in claim 1 in which said first variablemetering orifice comprises a first valve spool axially slidable to varythe size of said first metering orifice.

3. An apparatus as claimed in claim 2 which includes a piston responsiveto first and second servo operating pressure dependent on the enginecompressor pressures, said piston being operatively connected to saidfirst valve spool.

4. An apparatus as claimed in claim 3 which includes a pilot valvearrangement responsive to said compressor pressures to control therelative magnitudes of said servo operating pressures and meansconnecting said first valve spool to said pilot valve arrangement so asto urge the latter to vary said servo operating pressure to opposemovement of said first valve spool.

5. An apparatus as claimed in claim 4 which includes a bellows unitresponsive to an air pressure signal derived from the engine compressor,and said pilot valve arrangement includes a pair of closure membersmovable by said bellows unit to vary said first and second servooperating pressures respectively.

6. An apparatus as claimed in claim 1 in which the control members forsaid second and third orifices respectively comprise first and secondpiston elements slidable in response to said second operating parameter.

7. An apparatus as claimed in claim 6 in which each of said second andthird orifices includes a ported sleeve within which the associatedpiston element is movable.

8. An apparatus as claimed in claim 7 in which each said piston elementincludes a port which co-operates with the port in the associated sleeveto define said second and third variable metering orifices.

9. An apparatus as claimed in claim 6 which includes means for rotatingthe control member of said first metering orifice in response to saidsecond engine operating parameter.

10. An apparatus as claimed in claim 9 which includes athree-dimensional cam secured to the first metering orifice controlmember for movement therewith, and first and second cam followerelements movable by said cam and respectively coacting with said pistonelements to vary the sliding positions thereof.

11. An apparatus as claimed in claim 10 in which said piston elementsare respectively responsive to third and fourth servo operatingpressures, and each said piston element includes valve means responsiveto the relative positions of the piston element and the associated camfollower element to vary the magnitude of the corresponding servooperating pressure.

12. An apparatus as claimed in claim 6 which includes means for rotatingsaid piston elements in response to said third operating parameter.

13. An apparatus as claimed in claim 12 in which said piston elementrotating means comprises an edge cam rotatable in response to said thirdoperating parameter, and two cam follower assemblies biased into engagement with said edge cam and secured to said respective piston elements.

14. An apparatus as claimed in claim 1 in which sai second engineoperating parameter comprises the temperature at the engine compressorintake.

15. An apparatus as claimed in claim 1 in which said third engineoperating parameter comprises the position of a power demand control forthe engine.

16. An apparatus as claimed in claim 1 in which said throttlesassociated with said first, second and third metering orificesrespectively include third, fourth and fifth piston elements.

17. An apparatus as claimed in claim 16 in which said third, fourth andfifth piston elements are respectively responsive to an increase in thepressures downstream of the associated metering orifices to open saidthrottles, and are also responsive to increases in said first, secondand third servo pressure signals respectively to shut said throttles.

18. An apparatus as claimed in claim 17 in which said servo pressurecontrol arrangement comprises a second valve spool movable in responseto the difference between the pressures downstream of the first andsecond metering orifices, and a third valve spool movable in response tothe difference between the pressures downstream of the first and thirdmetering orifices.

19. An apparatus as claimed in claim 18 in which said second servopressure signal is responsive to the position of said second valvespool, an increase in the pressure downstream of said metering orificeor a decrease in the pressure downstream of said first metering orificeacting to decrease said second servo pressure signal.

20. An apparatus as claimed in claim 18 in which said third servopressure signal is responsive to the position of said third valve spool,an increase in the pressure downstream of said third metering orifice ora decrease in the pressure downstream of said first metering orificeacting to decrease said third servo pressure signal.

21. An apparatus as claimed in claim 18 in which said second and thirdvalve spools are movable to increase said first servo pressure signalwhen the difference between the pressures downstream of the first andsecond metering orifices or the difference between the pressuresdownstream of the first and third metering orifices exceeds apredetermined value.

22. An apparatus as claimed in claim 1 which includes regulating meansfor maintaining the pressure drop across the first metering orificesubstantially constant for any given engine speed.

23. An apparatus as claimed in claim 22 in which said regulating meanscomprises a fuel pump driven, in use, at a speed proportional to thespeed of the engine, and valve means responsive to the delivery pressureof the pump, to the engine speed, and to the pressure downstream of saidfirst metering orifice, to vary said pump delivery pressure.

24. An apparatus as claimed in claim 23 in which said valve meanscomprises a control valve responsive to a fourth servo pressure signalto vary the flow through the pump.

25. An apparatus as claimed in claim 24in which said regulating meansincludes a fourth valve spool responsive to an increase in said pumpdelivery pressure to reduce said fourth servo pressure signal, and to adecrease in the pressure downstream of said first metering orifice or toan increase in engine speed to increase said fourth servo pressuresignal.

26. An apparatus as claimed in claim 6 which includes athree-dimensional cam rotatable in response to said second engineoperating parameter, and first and second cam follower elements movableby said cam and respectively coating with said piston elements to varythe sliding positions thereof.

27. An apparatus as claimed in claim 26 in which said piston elementsare respectively responsive to third and fourth servo operatingpressures, and each said piston element includes valve means responsiveto the relative positions of the piston element and the associated camfollower element to vary the magnitude of the corresponding servooperating pressure.

28. An apparatus as claimed in claim 26 which includes means for movingsaid three-dimensional cam axially in accordance with the position ofthe control member of said first metering orifice and also in accordancewith said third operating parameter.

29. An apparatus as claimed in claim 28 in which said means for axiallymoving said cam comprises a quadrilateral linkage having one arm movableabout a first pivotal axis by movement of the control member of saidfirst metering orifice, an arm adjacent said one arm and pivotallymovable about said first pivotal axis to cause axial movement of saidcam, and means for varying the locus of movement of second pivotal axisof said linkage in accordance with said third parameter, said secondaxis being opposite said first axis.

30. An apparatus as claimed in claim 29 in which said locus varyingmeans comprises a crank rotatable about a third axis in accordance withsaid third parameter and a link interconnecting said second pivotal axisand the end of the crank remote from said third axis.

31. An apparatus as claimed in claim 29 which said arms of said linkageeach include a gear sector centred about said first pivotal axis, saidgear sectors meshing with respective raks which respectively coact withsaid first metering orifice control member and said threedimensionalcam.

1. A fuel control apparatus for a gas turbine engine reheat system whichincludes a plurality of burners, comprising a first variable meteringorifice including a control member movable to control fuel flow to afirst reheat burner in accordance with a first engine operatingparameter, second and third variable metering orifices to control fuelfLow to second and third reheat burners respectively, said second andthird orifices each including a control member movable in response tothe position of the control member of the first metering orifice andalso in response to second and third engine operating parameters,throttles between the metering orifices and their associated burners,the throttle associated with the first metering orifice being responsiveto a first servo pressure signal dependent on the pressures downstreamof both the second and third metering orifices, and the throttlesrespectively associated with the second and third metering orificesbeing responsive to second and third servo pressure signals respectivelydependent on the pressures downstream of the associated meteringorifices, and an arrangement for controlling said servo pressure signalsin accordance with said downstream pressures and with the speed of theengine, said arrangement having an internal ring gear, a first spur geardriven, in use, by the engine and meshing with the ring gear, a speedresponsive governor driven by the ring gear, a valve spool rotatable bythe ring gear and axially movable by the governor, a second spur gearsecured coaxially to the ring gear, a plurality of further valve spoolsrespectively axially movable in response to said downstream pressuresand a plurality of further spur gears respectively secured to saidfurther spools and meshing with said second spur gear.
 2. An apparatusas claimed in claim 1 in which said first variable metering orificecomprises a first valve spool axially slidable to vary the size of saidfirst metering orifice.
 3. An apparatus as claimed in claim 2 whichincludes a piston responsive to first and second servo operatingpressure dependent on the engine compressor pressures, said piston beingoperatively connected to said first valve spool.
 4. An apparatus asclaimed in claim 3 which includes a pilot valve arrangement responsiveto said compressor pressures to control the relative magnitudes of saidservo operating pressures and means connecting said first valve spool tosaid pilot valve arrangement so as to urge the latter to vary said servooperating pressure to oppose movement of said first valve spool.
 5. Anapparatus as claimed in claim 4 which includes a bellows unit responsiveto an air pressure signal derived from the engine compressor, and saidpilot valve arrangement includes a pair of closure members movable bysaid bellows unit to vary said first and second servo operatingpressures respectively.
 6. An apparatus as claimed in claim 1 in whichthe control members for said second and third orifices respectivelycomprise first and second piston elements slidable in response to saidsecond operating parameter.
 7. An apparatus as claimed in claim 6 inwhich each of said second and third orifices includes a ported sleevewithin which the associated piston element is movable.
 8. An apparatusas claimed in claim 7 in which each said piston element includes a portwhich co-operates with the port in the associated sleeve to define saidsecond and third variable metering orifices.
 9. An apparatus as claimedin claim 6 which includes means for rotating the control member of saidfirst metering orifice in response to said second engine operatingparameter.
 10. An apparatus as claimed in claim 9 which includes athree-dimensional cam secured to the first metering orifice controlmember for movement therewith, and first and second cam followerelements movable by said cam and respectively coacting with said pistonelements to vary the sliding positions thereof.
 11. An apparatus asclaimed in claim 10 in which said piston elements are respectivelyresponsive to third and fourth servo operating pressures, and each saidpiston element includes valve means responsive to the relative positionsof the piston element and the associated cam follower element to varythe magnitude of the corresponding servo operating pressure.
 12. Anapparatus as claimed in claim 6 which includes means for rotating saidpiston elements in response to said third operating parameter.
 13. Anapparatus as claimed in claim 12 in which said piston element rotatingmeans comprises an edge cam rotatable in response to said thirdoperating parameter, and two cam follower assemblies biased intoengagement with said edge cam and secured to said respective pistonelements.
 14. An apparatus as claimed in claim 1 in which said secondengine operating parameter comprises the temperature at the enginecompressor intake.
 15. An apparatus as claimed in claim 1 in which saidthird engine operating parameter comprises the position of a powerdemand control for the engine.
 16. An apparatus as claimed in claim 1 inwhich said throttles associated with said first, second and thirdmetering orifices respectively include third, fourth and fifth pistonelements.
 17. An apparatus as claimed in claim 16 in which said third,fourth and fifth piston elements are respectively responsive to anincrease in the pressures downstream of the associated metering orificesto open said throttles, and are also responsive to increases in saidfirst, second and third servo pressure signals respectively to shut saidthrottles.
 18. An apparatus as claimed in claim 17 in which said servopressure control arrangement comprises a second valve spool movable inresponse to the difference between the pressures downstream of the firstand second metering orifices, and a third valve spool movable inresponse to the difference between the pressures downstream of the firstand third metering orifices.
 19. An apparatus as claimed in claim 18 inwhich said second servo pressure signal is responsive to the position ofsaid second valve spool, an increase in the pressure downstream of saidmetering orifice or a decrease in the pressure downstream of said firstmetering orifice acting to decrease said second servo pressure signal.20. An apparatus as claimed in claim 18 in which said third servopressure signal is responsive to the position of said third valve spool,an increase in the pressure downstream of said third metering orifice ora decrease in the pressure downstream of said first metering orificeacting to decrease said third servo pressure signal.
 21. An apparatus asclaimed in claim 18 in which said second and third valve spools aremovable to increase said first servo pressure signal when the differencebetween the pressures downstream of the first and second meteringorifices or the difference between the pressures downstream of the firstand third metering orifices exceeds a predetermined value.
 22. Anapparatus as claimed in claim 1 which includes regulating means formaintaining the pressure drop across the first metering orificesubstantially constant for any given engine speed.
 23. An apparatus asclaimed in claim 22 in which said regulating means comprises a fuel pumpdriven, in use, at a speed proportional to the speed of the engine, andvalve means responsive to the delivery pressure of the pump, to theengine speed, and to the pressure downstream of said first meteringorifice, to vary said pump delivery pressure.
 24. An apparatus asclaimed in claim 23 in which said valve means comprises a control valveresponsive to a fourth servo pressure signal to vary the flow throughthe pump.
 25. An apparatus as claimed in claim 24 in which saidregulating means includes a fourth valve spool responsive to an increasein said pump delivery pressure to reduce said fourth servo pressuresignal, and to a decrease in the pressure downstream of said firstmetering orifice or to an increase in engine speed to increase saidfourth servo pressure signal.
 26. An apparatus as claimed in claim 6which includes a three-dimensional cam rotatable in response to saidsecond engine operating parameter, and first and second cam followerelements movable by said cam and respectively coating with said pistonelements to vary the sliding positions thereof.
 27. An apParatus asclaimed in claim 26 in which said piston elements are respectivelyresponsive to third and fourth servo operating pressures, and each saidpiston element includes valve means responsive to the relative positionsof the piston element and the associated cam follower element to varythe magnitude of the corresponding servo operating pressure.
 28. Anapparatus as claimed in claim 26 which includes means for moving saidthree-dimensional cam axially in accordance with the position of thecontrol member of said first metering orifice and also in accordancewith said third operating parameter.
 29. An apparatus as claimed inclaim 28 in which said means for axially moving said cam comprises aquadrilateral linkage having one arm movable about a first pivotal axisby movement of the control member of said first metering orifice, an armadjacent said one arm and pivotally movable about said first pivotalaxis to cause axial movement of said cam, and means for varying thelocus of movement of second pivotal axis of said linkage in accordancewith said third parameter, said second axis being opposite said firstaxis.
 30. An apparatus as claimed in claim 29 in which said locusvarying means comprises a crank rotatable about a third axis inaccordance with said third parameter and a link interconnecting saidsecond pivotal axis and the end of the crank remote from said thirdaxis.
 31. An apparatus as claimed in claim 29 which said arms of saidlinkage each include a gear sector centred about said first pivotalaxis, said gear sectors meshing with respective raks which respectivelycoact with said first metering orifice control member and saidthree-dimensional cam.